U.S. patent number 8,032,073 [Application Number 11/800,299] was granted by the patent office on 2011-10-04 for satellite communication with multiple active gateways.
This patent grant is currently assigned to STM Networks, Inc.. Invention is credited to Richard R. Forberg, Hans Peter Lexow, Emil Youssefzadeh.
United States Patent |
8,032,073 |
Youssefzadeh , et
al. |
October 4, 2011 |
Satellite communication with multiple active gateways
Abstract
Methods and apparatus are disclosed to enable a fixed or a
mobile ground based slave stations (VSAT: Very Small Aperture
Terminal) in a communication network to receive TDM transmissions
from and transmit TDMA burst transmissions to one or more
ground-based gateway stations in a networking system that employs
one or more geosynchronous satellites. Each gateway station
transmits on one or more forward TDM channels to the slave
stations; however, one primary gateway acts as the master station
at any given time which transmits the network control messages to
the slave stations (VSATs) that control their TDMA transmission
behavior on the network.
Inventors: |
Youssefzadeh; Emil (Palos
Verdes Estates, CA), Lexow; Hans Peter (Oslo, NO),
Forberg; Richard R. (Windham, NH) |
Assignee: |
STM Networks, Inc. (Irvine,
CA)
|
Family
ID: |
39101280 |
Appl.
No.: |
11/800,299 |
Filed: |
May 3, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20080043663 A1 |
Feb 21, 2008 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
60746356 |
May 3, 2006 |
|
|
|
|
Current U.S.
Class: |
455/3.02; 455/98;
370/321; 455/12.1; 370/316; 455/427 |
Current CPC
Class: |
H04B
7/2048 (20130101); H04B 7/18528 (20130101) |
Current International
Class: |
H04H
20/74 (20080101) |
Field of
Search: |
;455/3.02,427,21.1,98,12.1 ;370/316,321 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Anderson; Mathew D.
Assistant Examiner: Aminzay; Shaima Q
Attorney, Agent or Firm: Arjomand; Ataullah
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION(S)
This application claims the benefit of U.S. Provisional Patent
Application No. 60/746,356, filed on May 3, 2006.
Claims
We claim:
1. A method of communication of a slave station with more than one
gateway stations in a Time Division Multiple Access (TDMA) or Multi
Frequency-Time Division Multiple Access (MF-TDMA) satellite
network, wherein one gateway station is a primary gateway station,
the method comprising: receiving information, by the slave station,
from an assigned Time Division Multiplexing (TDM) channel
transmitting network control information; joining an assigned TDMA
or MF-TDMA network by employing a received information from the
assigned TDM channel and logging on to an assigned primary gateway;
getting permission and required information from the primary
gateway station to communicate with at least one other gateway
station, when the primary gateway station is so configured to
recognize communication capabilities of the slave station, or when
the slave station is so preconfigured to attempt such communication
with the at least one other gateway station without the permission
of the primary gateway station; and communicating with the at least
one other gateway station by receiving from, transmitting to, or
receiving from and transmitting to the at least one other gateway
station, wherein the slave station having capability to transmit to
multiple gateway stations is configured to have TDMA burst
transmitters of the slave station aligned with TDMA burst receivers
at the other gateway station by employing a Global Positioning
System (GPS) timing reference and a Network Time Protocol client
such that the other gateway station is able to link its local
timing reference equipment closely to the network clock reference
timing of the primary gateway station.
2. A method of communication of a slave station with more than one
gateway stations in a Time Division Multiple Access (TDMA) or Multi
Frequency-Time Division Multiple Access (MF-TDMA) satellite
network, wherein one gateway station is a primary gateway station,
the method comprising: receiving information, by the slave station,
from an assigned Time Division Multiplexing (TDM) channel
transmitting network control information; joining an assigned TDMA
or MF-TDMA network by employing a received information from the
assigned TDM channel and logging on to an assigned primary gateway;
getting permission and required information from the primary
gateway station to communicate with at least one other gateway
station, when the primary gateway station is so configured to
recognize communication capabilities of the slave station, or when
the slave station is so preconfigured to attempt such communication
with the at least one other gateway station without the permission
of the primary gateway station; and communicating with the at least
one other gateway station by receiving from, transmitting to, or
receiving from and transmitting to the at least one other gateway
station, wherein accurate and frequently updated position
information, including longitude, latitude and altitude, about the
location of the slave station, the location of a satellite, the
location of the primary gateway station, and the location of the at
least one other gateway station are used to determine constantly
changing distances between the satellite, the slave station and the
primary gateway stations and derive accurate estimates of
time-of-flight differences for electromagnetic carrier frequencies
used by the assigned TDMA channel in the TDMA or MF-TDMA satellite
network and compensate for the time-of-flight differences.
3. A slave station in a Time Division Multiple Access (TDMA) or
Multi Frequency-Time Division Multiple Access (MF-TDMA) satellite
network with more than one gateway stations wherein one gateway
station in the network is a primary gateway station, the slave
station comprising: means for receiving information from an
assigned Time Division Multiplexing (TDM) channel transmitting
network control information; means for joining an assigned TDMA or
MF-TDMA network by employing the received information from the
assigned TDM channel and logging on to an assigned primary gateway;
means for getting permission and required information from the
primary gateway station to communicate with at least one other
gateway station, when the primary gateway station is so configured
to recognize communication capabilities of the slave station, or
when the slave station is so preconfigured to attempt such
communication with the at least one other gateway station without
permission of the primary gateway station; and means for
communicating with the at least one other gateway station by
receiving from, transmitting to, or receiving from and transmitting
to the at least one other gateway station.
Description
TECHNICAL FIELD
Disclosed embodiments relate, in general, to satellite
communication systems and, in particular, to TDM channel reception
from a master station (or "gateway" or "hub") by a ground based
slave station (VSAT), and TDMA or MF-TDMA methods for return
channel communication from a ground based slave station (VSAT) to a
master station or to another type of gateway station or hub or to
another slave station.
BACKGROUND
Satellite networking systems supporting two-way communications that
have one active ground station functioning as the master station
and a plurality of widely distributed slave stations are very
common today. These slave stations are often called "VSATs"--Very
Small Aperture Terminals--or simply "terminals." There are
international standards defining how such VSAT networks should be
built and operated. The most comprehensive and widely adopted
standard is the DVB-RCS standard which is a family of DVB (Direct
Video Broadcast) standards developed by the DVB Project and
published by the European Technical Standards Institute (ETSI). See
ETSI EN 301 790 and www.etsi.org.
The DVB-RCS standard utilizes TDM (Time Division Multiplexing) on
the forward channel to the VSATs, and MF-TDMA (Multi-Frequency Time
Division Multiple Access) techniques on the return channels to the
master station. Most such VSAT networks today--even those not based
on the DVB-RCS standard--use TDM and MF-TDMA techniques in a
similar way as described in the DVB-RCS standard, though particular
details of their implementations may differ. Some older technology
VSAT networks may still use a single return channel (at a single
carrier frequency) and, therefore, only employ TDMA.
The embodiments disclosed herein apply to any type of VSAT network
that utilizes TDM communications from the master station and either
MF-TDMA or simply TDMA techniques on the return channel
communications to the master station in what forms a star topology
network with the master at the hub. They also apply to situations
where slave stations or VSATs may be able to communicate to each
other directly by using TDMA communications on one or more assigned
channels, in what forms a mesh topology network among the slave
stations, which is overlaid on a star-topology network. Both
situations are common today. However, these embodiments are mere
examples and do not limit the invention to these specific
communication types.
VSAT networks are used for providing two-way data, voice, and/or
video communication between one major location, such as near a
metropolitan area or an Internet backbone site, and a variety of
more remote locations, such as small businesses or homes in
suburban or rural areas or entire villages or towns in remote areas
of some countries. Such networks are particularly useful in areas
where the terrestrial telecommunication infrastructures are less
developed than those commonly found in major cities of well
developed countries. They are also useful as a low-cost competitive
alternative to many terrestrial services.
Today, because all VSAT network technologies only allow one active
master station, their flexibility is limited. The master station
usually also functions as the "gateway" between the VSAT slave
stations, which are often isolated, and the rest of world's
telecommunications infrastructure. (A master station or a gateway
station sometimes is also called a "hub station" because of its
role as the hub of a star topology network.) Therefore, a desired
enhancement to VSAT networking technology of all types is to allow
communication with multiple gateways with any VSAT station of the
network. Such multi-gateway enhancement has applications and
advantages of the following general nature: a.) A data
communication VSAT network, using the Internet Protocol (IP) or
other data protocols, can direct traffic targeted for a first data
processing center to a first gateway and traffic targeted for a
second data processing center to a second gateway. b.) A voice
communication VSAT network, such as Voice over IP or via other
means, can direct all calls within the country to a first gateway
and all calls to international destinations to a second gateway.
c.) In case of the failure or destruction of a first gateway
station a second gateway in the network, distant from the first,
can (dynamically, automatically, and/or based on a pre-defined
routing procedure, etc.) may be able to take over the role of
master and provide additional reliability not available in current
VSAT networks.
However, special considerations are required to enable the slave
station (VSAT) to receive TDM transmission from multiple gateways
and to send TDMA transmission to multiple distinct gateways. That
is because the gateway station--as noted earlier--also acts as the
master station for the entire VSAT network and it is not possible
for a VSAT network to have two or more master stations operating
concurrently.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a typical TDM/TDMA communication network with a
master station using a geosynchronous satellite to communicate with
a plurality of slave stations, where such network may also use
multiple TDMA channels and thus be MF-TDMA capable.
FIG. 2 illustrates a concept of alignment in TDMA burst
transmissions from the slave stations to the master station.
FIG. 3 illustrates a TDM/TDMA communication network using a
geosynchronous satellite that has one master station (also acting
as a primary gateway) and a secondary gateway communicating with a
plurality of slave stations (VSATs).
FIG. 4 shows a TDM/TDMA satellite communication network with one
master station acting as a primary gateway and two secondary
gateways.
FIG. 5 shows common elements of a typical master station in a
modern TDM/TDMA satellite network which also acts as a gateway to
the Internet or a WAN.
FIG. 6 illustrates a modified secondary gateway station so as not
to transmit its network control signals to the slave stations
(VSATs) over its TDM channel(s).
FIG. 7 shows a typical slave station (VSAT) in a TDM/TDMA network
supporting only star topology networking with the master
station.
FIG. 8 shows a slave station (VSAT) that has mesh topology
networking capabilities.
FIG. 9 shows a slave station (VSAT) supporting only star-topology
networking having multiple TDM receivers to receive the
communications of multiple TDM channels from multiple secondary
gateway stations, in addition to one or more TDM channels from the
master station or primary gateway.
FIG. 10 shows a slave station (VSAT) supporting both star-topology
and mesh-topology networking having multiple TDM receivers to
receive the communications of multiple TDM channels from multiple
secondary gateway stations, in addition to one or more TDM channels
from the master station or primary gateway.
FIG. 11 shows a logical layer process of a typical slave station
(VSAT) with one TDM receiver.
FIG. 12 shows a logical layer processing of a slave station (VSAT),
where there are multiple TDM receivers but only one TDM receiver is
receiving the TDM channel from the master that contains the network
control information, which may also carry user traffic.
FIG. 13 shows the logical layer processing of a slave station
(VSAT) that has mesh networking capabilities, where there are
multiple TDM receivers to receiver multiple channels, but only one
TDM receiver is receiving the TDM channel from the master that
contains the network control information, which may also carry user
traffic.
FIG. 14 shows a secondary gateway or master-capable station
enhanced with multiple TDM receivers for user traffic or for
special control traffic need to coordinate among secondary gateways
or master capable stations, as well as a current master
station.
DETAILED DESCRIPTION
The following description provides specific details for a thorough
understanding of, and enabling description for, embodiments of the
invention. However, one skilled in the art will understand that the
invention may be practiced without these details. Well known
structures and functions have not been shown or described in detail
in order to avoid obscuring the description of the embodiments of
the invention.
Disclosed embodiments present methods and apparatus for enabling a
fixed or a mobile ground-based slave station (VSAT) in a TDMA or
MF-TDMA network receive multiple continuous mode TDM transmissions
from and transmit TDMA burst transmissions to one or more
concurrently active ground-based gateway stations in a digital data
networking system that employs one or more geosynchronous
satellites, where each gateway station transmits on one or more
forward channels, utilizing TDM techniques, but where one primary
gateway station has the unique role of the master station at any
given time and thus only one gateway station transmits network
control messages to the slave stations (VSATs).
Notes on Terminology
A master station need not actually act as a gateway between the
slave stations and the terrestrial network, neither may it be the
sole communications hub in a star topology network. This is because
some TDMA and MF-TDMA satellite networks may allow for partial or
full mesh communications among the slave stations (without the need
of those communications passing through the master station), and in
such a mesh satellite communication network any one of the slave
stations may in fact act as a limited gateway into the world's
terrestrial networks on behalf of other slave stations that may
lack such terrestrial connections. However, because the TDM
continuous mode transmission and reception is more efficient and
costs less to implement in hardware than TDMA burst mode
transmission and reception, a gateway station resembling a master
station in capabilities will generally be a more effective and
efficient high-throughput gateway for high-volume traffic coming
from the world's terrestrial telecommunications infrastructure
going to the slave stations.
Likewise the use of "VSAT" or "terminal" (in place of "slave
station") may be misleading because it obscures their fundamental
role as slaves in the network, and implies that they are merely
end-points in the network and/or earth stations with very small
antennas, neither of which may actually be the case in the
disclosed embodiments.
Therefore, throughout this detailed description, a clear
distinction will be made between the role of master station vs.
that of gateway station. Furthermore the term "slave station" will
be used in place of "VSAT" or "terminal", because a "slave station"
may neither be a communications end-point in the satellite network
nor use a very small diameter antenna.
Also, for simplicity, reference to both TDMA and MF-TDMA methods
will be made as "TDMA," except where it is of particular importance
to distinguish those situations where multiple return to the master
and possibly other TDMA channels are enabled, each at a different
frequency, that can be used by the slave terminals (e.g., by
frequency hopping) for communications with the master station or
directly with other slave stations.
Types of TDMA Networks
TDMA networks (whether satellite-based or purely terrestrial-based
network) may be static, quasi-static, or dynamic. In a static TDMA
network, each slave station is given a fixed bandwidth to transmit
to the master station. In quasi-static TDMA networks, the operator
may change the return channel bandwidth allocated to a given
station. In dynamic TDMA networks the slave stations may request
bandwidth as needed for transmitting user traffic, and can be
assigned that bandwidth dynamically and very quickly, within some
business-oriented or policy guidelines and/or within the
constraints of the technology. Most modern wide-area TDMA networks
are dynamic, since it creates the greatest potential for the
efficient use of bandwidth, which for "over the air" networks is
usually scarce and expensive. And most modern TDMA networks are in
fact MF-TDMA in nature, because they support multiple TDMA
channels, each on a different carrier frequency, where slave
stations may use frequency hopping to move between these TDMA
channels and gain access to additional bandwidth and flexibility.
However, it is very uncommon for the TDM (or forward channel)
capacity in a TDMA network to be dynamically shared among multiple
gateways and the master station.
One common and early form of dynamic TDMA network technology is
known as "slotted aloha". In slotted aloha slave stations are
allowed to transmit randomly in any time slot, within some policy
constraints. There is no attempt to coordinate the transmissions of
different slave stations, thus there is a chance of collisions
among bursts from different slave stations that grow dramatically
with the level of congestion on the network. More modern TDMA and
MF-TDMA networks limit the use of slotted aloha techniques to just
those infrequent occasions where it is necessary (e.g., at time of
initial logon by the slave station to the network), so as to reduce
the adverse impacts on bandwidth efficiency caused by this access
technique. This is done by using control messages communicated to
slave stations by the master station.
Role of the Master Station in a TDMA Network
The master station plays a unique and critical role in a TDMA
network, whether it is satellite-based or purely terrestrial-based
network. The master station provides the network clock reference
information and a variety of timing correction messages, and other
control messages, to all the slave stations in the network. These
messages are essential to align the timing of the burst
transmissions from the slave stations, whether those transmissions
are addressed to the master station (for star topology operation),
or between any two slave stations in the network (for a mesh
topology operation).
If two or more different master stations were to attempt to provide
concurrently the network clock reference information and the
associated timing correction messages and other control messages
independently of each other, then inconsistencies, errors and/or
malfunctions would eventually occur or would be very difficult to
avoid. Thus there is only one active master station in a TDMA
network. This means the entire network is vulnerable and all slave
stations will be unable to either send or receive traffic if the
master station should fail. Present options for implementing
"back-up" master stations are very limited in their ability to
quickly take over control of the network.
The essential character of a TDMA network is that the slave
stations share one or more transmission channels with each other.
Each such transmission channel has a dedicated frequency band for
its carrier frequency which is modulated in some fashion
individually by each slave station. Each such transmission
channel--to support the use of TDMA techniques--must be divided
into a series of logical time slots during which transmission is
allowed by one and only one slave station at a time, except for the
possible presence of specially designated time slots that allow
"random access" (e.g., slotted aloha) by any slave station without
prior assignment from the master station.
When a slave station transmits on any of the available shared TDMA
channels, it must transmit in a short burst, that should (if the
slave station and the network overall are operating properly) fall
entirely into the assigned time slots for that slave station. Thus
slave stations posses what are called "burst transmitters", and
master station possesses what are called "burst receivers". Burst
transmitters are different in character from the continuously
modulated transmitters used by the master station to transmit the
TDM forward channel, in that burst transmitters must be able to
turn-on and turn-off their transmitter very quickly (e.g., for a
millisecond or less). Burst receivers for TDMA burst transmissions
are likewise different from ordinary receivers used to demodulate
continuously modulated TDM transmissions. Burst transmitters and
burst receivers are often frequency agile, meaning they can change
the carrier frequency they use very quickly, and perform what is
called "frequency hopping".
In a modern high speed TDMA network of any type--satellite based or
terrestrial--some of the allowed time slots may be only one
millisecond or smaller in duration. Thus TDMA networks impose very
precise timing alignment requirements upon slave stations so that
when one slave station is using its burst transmitter, in
accordance with timing instructions from the master stations,
another slave station does not start its burst before the current
slave station finishes. If this should happen, the burst
transmissions of one or both stations will be corrupted and
transmission errors will occur. Likewise, precise timing alignment
is important so that bandwidth resources are not unnecessarily
wasted by imposing undesirably large "guard times" around each time
slot. However, to a small degree, such guard times are necessary
because an absolutely perfect alignment of the timing among all
slave stations for their burst transmissions is impractical to
achieve in any TDMA technology.
Essential to a TDMA network is the time slot structure of each TDMA
channel. This structure defines the precise duration of each time
slot, including guard times. This may be a static and cyclical
structure in simple TDMA networks, or a dynamically changing
structure in more advanced TDMA networks. If it is a dynamically
changing structure, each new variation of it is communicated to the
slave stations periodically or on irregular basis, by the master
station. The basic slot structure, the allowed usage of each slot,
and instructions regarding which slave station may use which slot
for what purpose are communicated to slave stations by the master
station in what is commonly called the "burst plan" for the
network, which may change frequently as just noted.
The slave stations will have their burst transmitters aligned with
each other if and only if the time slot structure of the different
channels of the TDMA network for the upcoming interval of time, as
understood by each slave station, is aligned in such a way that if
each slave station were to transmit in a different time slot and
all time slots were occupied with bursts, no two bursts would
overlap or interfere upon reception at the master station.
Obtaining this alignment is not trivial given the differing
time-of-flight delays, and the possibly differing transmission
processing delays, associated with each different slave
station.
The Challenges of Satellite-Based TDMA Networks
The use of TDMA in satellite communication systems is very common
today and of growing importance. For many networking applications
it has rapidly replaced a simpler but less bandwidth efficient
approach known as "Single Channel Per Carrier", where one or more
dedicated carrier frequencies are allocated to each ground station
for its transmissions.
The implementation of TDMA in a satellite network (vs. a small
terrestrial-only, wireless or wired network) however, is
complicated by several factors: a.) The large geographic extent of
the network on or near the surface of the earth, typically
continental in extent covering many millions of square kilometers;
b). The large and differing distances between the satellite and
each of the different ground stations; c.) A potential mixture of
fixed and mobile ground stations, and d.) The fact that the
satellite in the sky is typically in motion relative to the surface
of the earth and hence also moving relative to all the ground
stations.
Even a geosynchronous satellite--which is approximately 35,800
kilometers above the equator of the earth at any one of various
longitudes along the equator of the earth, spaced approximately 2
degrees apart in longitude--typically undergoes periodic,
detectable and undesirable motion about its nominal "fixed"
position on the order of up to 50 kilometers.
The geosynchronous satellite obviously plays an important role in a
satellite communication network. It may possibly: (a) regenerate
signals from ground station transmissions and (b) switch either
through IF (intermediate frequencies) or baseband signals to one or
more other transponders on the satellite. But in most cases
satellites today do not do this. In all cases though, the
satellite: (a) amplifies the electromagnetic waves carrying
transmission signals it receives from the ground stations; (b)
extracts a modulated IF signal; (c) re-modulates a different
carrier frequency with that signal; and (d) re-directs the new
carrier frequency back to earth to reach additional ground
stations, which may--and in most cases does--include the
originating ground station of that signal.
The present embodiments consider satellites of all types mentioned
above. This does not result in notable variations in these
embodiments, because in none of these cases does the satellite play
any role in aligning the TDMA timing advances of the various slave
stations in a satellite network.
Ground stations working with satellites in geostationary orbit
typically use directive antennas to achieve high bit rates in both
transmission and reception, using power amplifiers of a reasonable
scale.
The positions on the surface of the earth where a satellite, or one
of its transponders, directs the electromagnetic waves it receives
are usually called the "footprint" of the satellite. They may also
be thought of as "beams of light" intersecting the surface of the
earth. The position on the surface of the earth from which a
satellite, or one of its transponders, can receive electromagnetic
waves from a ground station suitably positioned and pointed at it
without undue obstruction may also be thought of as being within
the foot print or beam of the satellite or one of its transponders.
All ground stations must be in the footprint (or beam) of the
satellite to receive signals from it, and to direct signals to it.
However, it must be noted that some ground stations may transmit
signals to one satellite (or transponder on a satellite) and
receive signals from a different satellite (or a different
transponder on the same satellite). Furthermore a ground station
may transmit to or receive from multiple satellites or multiple
transponders on the same satellite at the same time, if its antenna
and associated RF and baseband electronic are suitably configured.
The disclosed embodiments include these various common and less
common satellite networking arrangements.
In a modern high-speed TDMA satellite network the maximum variation
allowed in the timing alignment among slave stations (hence the
size of the guard times on certain time slots, particularly those
used for user traffic which comprise the majority of time slots
allocated to slave stations) may be less than a few microseconds.
Thus, assuming for illustration purposes a 5 microsecond guard time
is specified, a difference in the distance between one slave
station and the master station vs. other slave station and the
master station of only 1500 meters would be enough to necessitate a
mechanism in place for the master station to force each slave
station to correct its timing advance relative to the network clock
individually given its unique position on or near the surface of
the earth. (NOTE: This result is calculated simply from the speed
of light in air and free space which is approximately 300,000
kilometers per second and is the approximate speed at which all
electromagnetic waves travel in free space or air with some
variations depending on air densities and ion concentrations).
As noted above, in any TDMA network the timing alignment required
among the burst transmitters of the slave stations is most easily
understood as requiring alignment upon reception of those burst
transmissions at the master station where there exists the
necessary burst receiver technology for capturing, demodulating and
decoding each burst. Burst receivers must not only know the time
slot structure (e.g., type of slots, assigned function and duration
of each) used in the TDMA network for each channel, but must also
know when each different type of burst time slot on each different
TDMA channel is about to arrive (to within less than the size of
the smallest guard time used), and in more advanced TDMA networks,
like DVB-RCS, also know how each burst on each TDMA channel is
modulated and encoded by the slave station that sent it. This
effectively requires that burst receivers know which slave station
is using which specific (i.e., numbered) burst time slot, even
before the actual burst arrives at the master station. Thus it is
critical for the burst receivers to be fully aligned with the
detailed and constantly changing structure of burst plan and how it
is being used in all these respects, not just an alignment in a
simplified "relative time sense" like following the regular beat of
a drum.
In a TDMA network based on the use of a geosynchronous satellite,
it is useful to point out that if the burst transmitters among the
slave stations are aligned properly with the burst plan for
reception by the burst receivers of the master station, then the
burst transmitters are also aligned--relative to each other--upon
reception at the location of the applicable geosynchronous
satellite itself (.about.36,000 km above the surface of the earth).
That follows logically because the distance from the satellite to
the master station is the same for the incoming transmissions of
all slave stations using those same TDMA channels.
However, the distance between the satellite and the master station
is not the same at different points in time. That is because of the
motion of the satellite due to normal drift patterns or due to
intentional positional corrections by the satellite operator for
different reasons. The same applies for the distance between the
satellite and each slave station. Therefore the master station must
be able to frequently and individually adjust the timing advance
for each slave station even if all slave stations are fixed in
their locations. This is because even the slow or small drifts in
the motion of the satellite may unequally affect the distance
between the master station and the slave stations. Similarly, the
small amounts of satellite motion can affect the relative alignment
of the burst transmitters at the location of the satellite.
It is worth emphasizing that the applicable geosynchronous
satellite in this case is the one used for the carrier frequencies
of the applicable TDMA channel (or channels) from the slave
stations to the master station. If multiple TDMA channels are used
(as in MF-TDMA systems) and some are handled by one satellite and
others by a different satellite, which by necessity are in
different positions and undergo different motions at different
times, then the master station must be able to manage multiple and
distinct timing advances and adjustments, both for the satellites
and for the slave stations. Furthermore, the master station must
receive burst transmissions from all slave stations in the network,
sent regularly via each satellite corresponding to each slave
station, so that the master station can observe and compensate any
timing offset observed in the burst transmissions of any slave
station.
With this understanding of the unique role of the master station in
a TDMA satellite network, the essential background information
regarding the types of control messages transmitted by the Master
Station to the slave stations in the network is described
below.
Control Messages Transmitted by the Master Station to the Slave
Stations
Using its TDM forward channel, a master station transmits not only
the user data traffic (or user voice or video traffic) destined to
one or more of the slave stations but also control messages that
may be directed to one or all of the slave stations. Various ways
of coding these control messages, which are sometimes called
"signaling," may be used. Most TDMA networks use very efficient
coding techniques for constructing these control messages to
consume a minimal amount of bandwidth.
The control messages may be of various types and names and use
various encodings depending on the technological heritage and
applicable standards for the TDMA networking system of interest. In
general, though; in all modern TDMA networks there must be
sufficient signaling methods or messages to perform the following
functions: 1.) Network Clock Reference Messages. These are
broadcast messages transmitted to all slave stations. They carry a
counter value, which may be thought of as a clock ticking, so that
all slave station use the same clock. However because different
slave stations will likely receive these messages with widely
differing amounts of time-of-flight delay, the distribution of
these messages, by itself, does not guarantee that the burst
transmitters in all slave stations will be properly or adequately
aligned. They simply provide a common time reference frame from
which timing corrections/adjustments (i.e., timing advances) can be
made to align slave station burst transmitters with each other and
with the burst receivers at the master station. The farther a slave
station is from the master station the larger the timing advance
required for its local burst transmitter to insure that
transmission bursts from all slave stations will be aligned as they
return to the master station. In DVB-RCS technology these network
clock reference messages are called the Network Clock Reference
(NCR), which contain a counter value of very high accuracy. The NCR
in DVB-RCS systems is also a highly accurate piece of equipment
located at the master station, providing an NCR with better
accuracy than five parts per million (5 ppm) relative to
International Standard Organization (ISO) definition of one second,
the fundamental unit of time measurement in all scientific and
engineering work. Longer term clock accuracy of the NCR will be
better than one part in one billion (1 ppb). 2.) Burst Plan
Messages. These are messages that inform the slave stations of the
different types of burst time slots which they may use for
transmissions to the master station. Different types of slots are
used for various different functions, such as for requesting logon
to the network; sending of regular transmissions; sending capacity
request messages; and sending user traffic to the master station,
as well as possibly for sending various types of error reports or
status messages to the master station. The burst plan messages may
also inform a individual slave station which particular type of
slots it may use (unless the TDMA network is purely one of "slotted
aloha"), and may inform a stations about what type of modulation
and/or FEC encoding it should use for the that specific burst
transmission as well as, for MF-TDMA networks, which TDMA channels
to use for each burst transmission. A slave station in a modern
TDMA network must properly receive the burst plan message, prior to
transmitting anything to the master station, unless a special
dedicated channel, on a dedicated and known carrier frequency, is
provided just for logons. In that case a slave station may logon
prior to receiving and/or processing the burst plan information.
Most modern TDMA or MF-TDMA systems use special type of time slot,
rather than a dedicated channel for "logons" to the network by
slave stations. In DVB-RCS these are called Common Signaling
Channel (CSC) time slots, and may be accessed in a slotted aloha
manner (i.e., randomly). Typically, they are scheduled to occur
relatively infrequently within the burst time plan compared to
other types of time slots. This is because they are not needed very
often and such time slots must generally have much larger guard
times, for a slave station may not yet have adequately aligned in
its burst transmitter to other slave stations or the burst
receivers at the master station. 3.) Logon Response Message. This
is the message that master station sends back to the slave station
in response to the logon request. This message may contain a
variety of information necessary, but possibly not sufficient, for
the slave station to operate properly in the network. In DVB-RCS
technology, this message is typically also used to carry an initial
"large timing correction message" described below, as well as
session related information. 4.) Large Timing Correction Message.
This is a type of unicast message that tells an individual slave
station to make a large correction to its timing advance for its
burst transmitter. This type of message is necessary when the
master station detects that the burst transmissions from a specific
slave station are so unaligned that it would be outside of the
guard time allowed for other burst time slots. Thus it is of an
urgent and significant nature and must be processed by the slave
station with corresponding importance. Such messages are common
when a slave station first logs on to a TDMA or MF-TDMA network,
because the newly logged on slave station has not yet had it burst
transmitters finely aligned with the other slave stations. In
DVB-RCS systems this message is called the Correction Message
Descriptor. 5.) Small Timing Correction Message. This message
informs a slave station of the specific smaller correction it needs
to make in its individual timing advance setting. Thus they may be
unicast, multicast or broadcast messages depending on the specific
TDMA technology involved and their implementation details. It is
possible to send multiple such corrections for multiple slave
stations in one message. These are not as urgent, in that even
though the master station has detected a need for some corrections
in timing advance of these slave stations, the slave station is not
yet transmitting outside the allowed guard time for that type of
burst slot. In DVB-RCS, individual slave station messages of this
type are typically broadcasted in the Correction Message Table, but
may also be unicast as individual Correction Message Descriptors.
6.) Shut Down or Disable Messages. Most modern TDMA or MF-TDMA
networking systems also provide unicast control messages that force
a log-off of a slave station or a shut down of its burst
transmitter. These are useful and necessary when a slave station is
malfunctioning or repeatedly transmitting outside of the guard
times allowed, and therefore causing collisions with the burst
transmissions of other slave stations. 7.) Other Types of Messages.
There may be many other management and control functions sent to
the slave stations within any given type of TDMA or MF-TDMA
technology, e.g., various type error reports or signal strength
reports sent to the master station, but these are not directly
relevant to the presented embodiments.
It is important to note that not all TDMA satellite networks have
distinct messages types or distinct signaling methods for each of
the above functions. Some networks may not even support all of
these functions. However, the distinct or not so distinct character
of these different types of messages and signaling is immaterial to
the embodiments of the present invention. Neither is it required
that a given TDMA satellite technology support all of these
functions to implement the presented embodiments.
It is also important to note that a given TDMA technology may
implement these messages, or signaling methods, in a variety of
ways, e.g., in dedicated TDM time slots, and with various types of
Layer 2 framing, e.g., MPEG framing, ATM framing, or any other
types of framing at Layer 2, which allow the messages to be
directed to one, multiple, or all slave stations.
Management and Control Messages Transmitted by the Slave
Stations
Most modern TDMA and MF-TDMA networks support a variety of
management and control messages sent to the master station by the
slave stations. These were alluded to above, when discussing the
types of burst time slots supported in the burst plan messages. The
common types of management and control message sent to the master
station that are relevant to the disclosed embodiments are: 1.)
Logon Request Message. This is the message that the slave station
sends to the master station to request to be logged on to the TDMA
network. Typically, the slave station must identify itself by some
unique unit identifier. In addition this message might also carry
information about the slave station's position on the surface of
the earth (longitude, latitude and possibly altitude) which will be
useful in helping the master stations to establish a proper timing
correction for that slave station. Note that these messages are
only for the logon of the slave stations themselves, and are not
equivalent to any human user log-on processes that may be required
after the station itself is logged on. 2.) Periodic SYNC Message.
This is a message that must be regularly sent to the master
stations by each slave station, while logged on, so that the master
station may monitor the status of their alignment with other slave
stations. In the DVB-RCS technology, this message is simply called
"SYNC" and is implemented by very small time slots, so as not to
waste bandwidth. The time slots for SYNC messages would typically
be given larger guard times than slots for user traffic because of
their function of helping with misalignments in the slave station's
timing advance. 3.) Capacity Requests Message. In modern dynamic
bandwidth on demand TDMA networks (i.e., those that do not use
slotted aloha for all user traffic) it is necessary for a slave
station to be able to request that the master station assign burst
time slots to it for the transmission of user traffic. Such
requests may be formed by the slave stations, and processed by the
master stations under various possible types of policy constraints
and algorithms. In the DVB-RCS technology, this is typically how
bandwidth is dynamically allocated to slave stations.
Problems Solved
As explained above, TDMA networks can have one active master
station, which may also be called a primary gateway. The
requirement for one, and only one, active master station within the
network follows from the need for just one single station to set
the network clock reference information for the entire network and
to transmit the essential timing correction messages and other
control messages to each slave station.
The ability to support additional ground stations (called
"secondary gateways") at various locations of the network, which
may have identical features and capabilities to the actual master
station ("primary gateway") but which do not act as masters, has
several benefits, including: 1.) The primary and secondary gateways
may be located in widely dispersed and different locations. This
enables the user traffic or digital content to be sent from various
physical locations directly to the slave stations, without any
intervening transport or other communication methods that might
otherwise be required if there is only one gateway on the network.
It also saves considerable bandwidth and overhead costs since the
traffic or content needs to be sent only once through a single
network; 2.) It allows user generated data, voice, and/or video
traffic, or any type of digital content to be sent from multiple
gateway stations (primary and secondary) via the TDM continuous
mode of transmission, which is more efficient than TDMA
transmission in the use of bandwidth; 3.) Continuous mode
transmitters used by gateway stations are lower in cost than burst
mode transmitters for the same modulation and encoding performance;
4.) A much greater quantity of traffic or digital content can be
directed to a given slave station by having various secondary
gateways operating concurrently with the primary gateway; and 5.)
Any one of the secondary gateways, if suitably equipped, can very
rapidly take over the role of primary gateway in an event that a
current primary gateway (master station) should fail.
Of course a necessary condition for realizing some of the benefits
mentioned above is that a slave station must be physically equipped
with the appropriate number of TDM receivers for the number of the
TDM channels they need to receive and the processing power
necessary to handle the additional digital data streams which are
sent to it by multiple gateways over TDM channels. Given trends in
digital electronics for the common type receiver chips used in
slave stations (e.g., DVB-S and DVB-S2) this is increasingly
possible at low cost. Even if not all slave stations are so
equipped with multiple TDM receivers, benefits such as the option
of sending traffic through multiple secondary gateways in addition
to the master station and the rapid failure recovery are still
achievable.
FIG. 1 illustrates a typical TDMA communication network with a
master station using a geosynchronous satellite to communicate with
a plurality of slave stations. The master station in this example
is also acting as the primary gateway to the terrestrial
telecommunications infrastructure for most of the slave stations,
by being connected to the Internet or other terrestrial wide-area
networks (for two-way voice, data and/or video communications). All
signals transmitted to the satellite are returned from satellite
within the applicable footprint of the satellite as shown by the
growing large arcs approaching the ground. A slave station may or
may not be connected to a wide area network. The master station
transmits to the slave stations using a continuously modulated TDM
communication channel which carries both user traffic and control
messages or signals, some individually as unicast transmissions,
and some collectively as broadcast transmissions. The slave
stations transmit back to the gateway using TDMA burst transmission
techniques, which require alignment in the burst transmitters of
the slave stations, and may require frequency hopping in those
burst transmitters to support MF-TDMA techniques. In a star
topology network only the master station has the ability to receive
the burst transmissions from the slave stations. In a mesh topology
network slave stations have the ability to receive burst
transmission from other slave stations. However, only the master
station has the special high-accuracy electronic timing equipment
necessary to provide the required network clock reference messages
to the slave stations and the ability to transmit control messages
to all the slave stations.
FIG. 2 illustrates the concept of alignment in the TDMA burst
transmissions from the slave stations to the master station within
one TDMA channel following the instructions given in the burst time
plan and using the timing advances and network clock references
given to them by the master station (which is not shown). In this
illustration the burst plan is shown as a number series of time
slots, and alignment occurs at the geosynchronous satellite used
for the TDMA channel. A grey time slot indicates that a slave
station has made a burst transmission during that time slot. There
are no burst collisions in this illustration and no guard times are
shown. For simplicity the Figure shows a simple burst plan with all
time slots of equal size, and it shows the slave stations as if
they were directly beneath the satellite (generally, each slave
will be at a different angle and different distance relative to the
satellite, depending on its position on the surface of the earth
and the distance will be greater than 36,000 km.). The numbers next
to each time slot, in this illustration, are referenced to future
network clock reference values--namely the times at which the
master station is prepared to receive these bursts (though
individual slot numbers are not explicitly carried with the time
slot or the burst). It should be noticed that each slave station in
this example receives each network clock reference message at a
different point of time, depending on their distances from the
broadcasting satellite and these slaves have similarly different
distances to the receiving satellite, and therefore each of these
slaves applies a different timing advance relative to its locally
constructed network time reference to schedule the transmission of
its bursts, such that all bursts will be properly aligned in the
allowed slots of the burst plan. This example illustrates a
situation where burst plan slots are about 1 millisecond in
duration, since the example shows that it requires about 120
milliseconds for the electromagnetic wave carrying the burst to
travel to a geosynchronous satellite if the slave station is
directly beneath the satellite at the equator. For simplicity a
very coarse time counter (and slot numbering) using 1 millisecond
intervals is shown. In a high-performance TDMA satellite network
the counter resolution would have to be several orders of magnitude
more granular.
FIG. 3 illustrates a TDMA communication network using a
geosynchronous satellite that has one master station (also acting
as a primary gateway) and a secondary gateway communicating with a
plurality of slave stations (VSATs). Even though the secondary
gateway may be capable of being the master station, because it has
the special electronic equipment (e.g., accurate timing sources)
and other necessary control capabilities (e.g., network control
computers and software) to act as a master station, it does not
perform those functions while acting as secondary gateway. It
transmits a continuously modulated TDM forward channel to the slave
stations that contain only user traffic. It also has the ability to
receive burst transmissions from slave stations on one or more TDMA
channels. To do this, however, it must have its burst receivers
aligned with the overall burst plan for the network.
FIG. 4 shows a TDMA satellite communication network with one master
station acting as a primary gateway and two secondary gateways. It
is straight forward to support a plurality of secondary gateways in
this fashion. However, there can only be one station acting as the
master station in this network.
FIG. 5 shows common elements of a typical master station in a
modern TDMA satellite network which also acts as a gateway to the
Internet or a WAN. After traffic in the form of Layer 3 data
packets--typically Internet Protocol (IP) packets--enters from (or
before it exits to) the Internet or WAN via a router, it may pass
through a LAN switch to separate the transmit (Tx) path from the
receive (Rx) path of traffic flow. On the transmit path Layer 3
packets are then encapsulated, typically using a standard
multi-protocol encapsulation method, and placed into Layer 2 frames
(e.g., MPEG frames in DVB-RCS systems) specified by the TDM/TDMA
network. This may involve fragmentation of the encapsulated packet,
since they are often longer than the size of the Layer 2 frames
used. The Layer 2 frames are multiplexed serially and then
submitted for Layer 1 FEC (Forward Error Correction) encoding and
digital modulation, typically using schemes such as QPSK or 8PSK.
This is performed by the TDM transmitters which operate in
continuous modulation mode. A TDM transmitter typically up-converts
the modulated signal to Intermediate Frequency (IF) signal. It then
submits it to the Radio Frequency (RF) subsystem for transmission.
If there are multiple TDM channels then multiple TDM transmitters
are required. The RF subsystem converts the IF signal to even
higher radio frequencies, amplifies the signal and transmits it to
the satellite via the antenna. On the receive path, the incoming
TDMA channel(s) are first received by the antenna, down converted
to IF and amplified by the RF subsystem. If there are multiple TDMA
channels, a number of them may be handled by a single wide-band
TDMA burst receiver, which demodulates and decodes the traffic
burst signals on each channel. These decoded Layer 2 frames are
then handled by the packet re-assembler which re-builds the
original packets. The many required interconnections for management
and control are shown as dotted lines. Layer 2 control messages,
including network clock reference messages, and any special
signaling for the slave stations are injected by the network
control and management system directly into the TDM transmitter,
but only on one TDM channel; Layer 3 management messages (e.g.,
SNMP) are injected into the packet encapsulator. Incoming control
messages from slave stations (e.g., logon requests, capacity
requests) are extracted by the TDMA burst receivers and passed
directly to the network control and management system. The network
clock reference equipment provides the accurate timing reference
information for the network, which may be connected to a GPS timing
reference and Network Time Protocol (NTP) client.
Slave Station (VSAT) Capable of Communicating with Multiple
Gateways
A slave station (VSAT) configured to concurrently and
bi-directionally communicate with multiple gateways is disclosed,
herein referred to as a multi-gateway enhanced slave station or
VSAT. In one embodiment it is configured such that it may be
implemented in any common TDM/TDMA networking technology, including
DVB-RCS standard technology. In another embodiment it is also
configured such that the multi-gateway enhanced slave station is
able to operate in a network concurrently with the master station
and slave stations comprising a typical TDM/TDMA network of the
technology type for which it is implemented.
FIG. 7 shows a typical slave station (VSAT) in a TDMA network
supporting only star topology networking with the master station.
It bears some resemblance to a master station, but it uses TDMA
bursts for transmission on one or more designated TDMA channels and
receives on one or more continuously modulated TDM channels from
the master stations. It extracts control messages--including
network clock reference messages and timing corrections--from the
designated TDM control channel, which may also carrier user
traffic, and uses these to align its burst transmitters with the
burst receivers at the master station, and to learn about the burst
plan for the network. The slave station constructs its own image of
the network clock reference based on the received network clock
reference broadcasted from the master. It uses this locally
constructed network clock reference as the basis for burst
transmission alignment, adjusted by, potentially, a nominal timing
advance or a specific master controlled timing advance applicable
for the state and operation of the slave station. If applicable,
the nominal advance offset is typically set to compensate for the
propagation delay between the geographical position of the slave
and the geographical position assumed for the network clock
reference, and propagation delay between the geographical position
of the slave and the geographical position to be assumed to require
alignment of bursts, nominally the position of the burst receiving
antenna at the satellite.
FIG. 8 shows a slave station (VSAT) that has mesh topology
networking capabilities. It is the same in most respects to a slave
station with only star topology networking, but in addition, it has
one or more burst receivers attached to the Intermediate Frequency
(IF) Rx cabling via a splitter, or other mechanisms, to receive the
carrier frequencies used for TDMA transmissions from other slave
stations.
The multi-gateway capable slave station apparatus is formed by
using a ground station having the same types of hardware and
software capabilities as is typically used in a slave station
(VSAT) for either star-topology or mesh-topology networking (see
FIG. 7 and FIG. 8) with geostationary satellites, as employed for
that particular type of TDMA (or MF-TDMA) satellite networking
technology, but with the following changes: a.) Additional TDM
receivers are attached to the Intermediate Frequency (IF) receive
side (Rx) of the slave station's RF and antenna subsystems. In one
embodiment each TDM receiver may be simply a semiconductor chip
with some minor support circuitry. It is also possible that two or
more TDM receivers for the type of Layer 1 modulation and encoding
used in the network be implemented on a single chip. (See FIG. 9
and FIG. 10, which are described below.) b.) One of these TDM
receivers sets its local oscillators to tune to a previously
configured carrier frequency (or frequency band) for the TDM
channel designated by that network as the master TDM channel, which
contains essential network control information transmitted by the
master station, particularly the network clock reference, burst
plan information, logon responses and any additional timing related
or timing correction information necessary for the slave station to
determine its appropriate timing advance needed to align its TDMA
burst transmitter with the burst receivers of the master station,
and to behave as a proper citizen on the network. The slave station
may--and it would in general--also receive traffic routing control
information over this TDM channel from the master station, so that
the slave station learns which gateway station (or other slave
station) it should direct certain classes of user traffic to, using
its MAC Layer (Layer 2) addressing and handling capabilities.
However, it is possible that traffic routing information may be
sent over supplementary TDM channels not used for basic network
control functions such as timing control, if they are addressed at
Layer 3 to the routing function in the slave station for it to
processes directly. (See FIG. 11 and FIG. 12, which are described
below) c.) In addition, it may have multiple TDMA burst
transmitters to support additional flexibility and capacity in its
communication with multiple gateways and other slave stations.
Normally TDMA burst transmitters are considerably more expensive
that TDM burst receivers, and normally slave stations receive more
traffic than they transmit, so this is not a requirement of a
multi-gateway enhanced slave station. However, it is a desirable
option in some networks, where some sites have large amounts of
traffic to transmit, but may not be able to justify having an
entire gateway system.
FIG. 9 shows a slave station (VSAT) supporting only star-topology
networking--as disclosed herein--having multiple TDM receivers so
that it may receive the communications of multiple TDM channels
(for user traffic only) from multiple secondary gateway stations,
in addition to TDM channel from the master station (or primary
gateway) which contains network control information. (In addition
it may have multiple TDMA burst transmitters to support additional
flexibility and capacity in its communication with multiple
gateways and other slave stations.)
FIG. 10 shows a slave station (VSAT) supporting both star-topology
and mesh-topology networking--as disclosed herein--having multiple
TDM receivers so that it may receive the communications of multiple
TDM channels (for user traffic only) from multiple secondary
gateway stations, in addition to TDM channel from the master
station (or primary gateway) which contains network control
information. For mesh networking the slave station (VSAT) also has
one or more TDMA burst receivers so that it may receive traffic
from other slave stations over TDMA channels. (In addition it may
have multiple TDMA burst transmitters to support additional
flexibility and capacity in its communication with multiple
gateways and other slave stations.)
FIG. 11 shows functional processes of a typical slave station
(VSAT) with one TDM receiver. In this case both network control
information and user traffic is received over the single TDM
channel from must come from the master station (or primary
gateway). Both user traffic and network management related
information generated by the slave station (such as logon requests,
capacity requests, error reports, etc.) are transmitted via the
station burst transmitters.
FIG. 12 shows functional processes of a slave station (VSAT) as
disclosed herein, where there are multiple TDM receivers (to
receiver multiple channels), but only one TDM receiver handles the
TDM channel from the master that contains the network control
information.
FIG. 13 shows the functional processes of a slave station (or VSAT)
as disclosed in this invention, that has mesh networking
capabilities, where there are multiple TDM receivers (to receiver
multiple channels), but only one TDM receiver handles the TDM
channel from the master that contains the network control
information.
Method of Log-on by Multi-Gateway Enhanced Slave Station (VSAT)
In some of the embodiments the slave station proceeds to log on to
the network as it normally would. This typically involves the slave
station providing a unique hardware address (typically its own
Layer 2 or MAC address) to the master station, and may required
additional customized passwords or keys to be entered into the
slave station prior to its initial logon attempt. If the log-on is
successful, the master station will inform the slave station of the
usual information it provides to all newly logged-on slave
stations. This information--as customary today--may include: a
different TDM channel to listen to of those which are transmitted
by the master station for its routine operation. This other
channel, if applicable, will become the active control channel for
the slave station on this network, and if so, the slave station
will discontinue listening on initially configured TDM channel
carrier frequency, and possibly then re-logon to the newly
designated TDM channel. In addition this typical information may
possibly include position information (e.g., longitude, latitude,
altitude) pertaining to the location of master station, the nominal
position of the satellite, and the position of the slave station
itself, to assist in aligning the timing of its burst transmitters.
Additionally, it may include timing correction messages (large and
small) for the same purpose.
The master station can identify the multi-gateway enhanced slave
stations by their Layer 2 address or another identifier configured
into the slave station for log-on, which the master station knows
to look for based on details of the implementation desired, which
can be easily devised by a person skilled in the art.
After successful log-on, the multi-gateway enhanced slave station
may also be provided, as disclosed herein, with certain additional
informational messages by the master station, containing
information such as the carrier frequencies modulation rates and
FEC encoding used by other gateways for the supplementary TDM
channels they transmit and the TDMA channels they receive; the
satellites used for these supplementary TDM and the various
additional TDMA channels and the nominal positions of the
satellites used for these channels; the nature of the informational
content services or interactive services available from those
gateways; their location, their hours of operation, etc., so that
the slave station may make the best use of the available other
gateways on the network and their supplementary TDM channels. If
this approach is not used then the multi-gateway enhanced slave
station may be preconfigured with this information or a sub-set of
it. These messages may be implemented in a variety of forms and
delivered via a variety of common or standard mechanisms, and may
be easily devised and implemented by a person skilled in the art,
either at Layer 2 or at Layer 3.
The master station may also securely transmit certain keys or
passwords to use on the network to gain access to certain other
gateway stations and or their supplementary TDM channels, or as
needed to transmit to those other gateways, or as may be needed to
decrypt content that is distributed by the master station or other
gateway stations over those supplementary TDM channels, as needed
to make the best use of other gateways and the available
supplementary TDM channels on the network. Such keys or passwords
can be distributed securely by using, for example, well-know
techniques involving Public Key Infrastructure (PKI) technology and
in messages similar to those devised above for distributing other
information to the multi-gateway enhanced slave stations.
These supplementary TDM channels may be used by secondary gateways
or the master for transmission of the outbound component of
interactive data, video conferencing or voice traffic, or the
transmission of broadcast or multicast one-way traffic. Interactive
traffic and/or digital content may be distributed in either
standard (e.g., MPEG) or other types of Layer 2 frames, or in Layer
3 data packets such as IP (and encapsulated into Layer 2 frames for
transport over the satellite network), or a combination of all of
these (e.g., interactive traffic at Layer 3 or Layer 2, Layer 2
broadcasts and multicasts, as well as Layer 3 broadcast and
multicast traffic), and the method of distribution may vary by
supplementary TDM channel.
The additional TDMA channels may be used by the enhanced slave
stations for transmission to the various other secondary gateway
stations, as well as for mesh communications with other slave
stations, if so allowed by the master station.
If the multi-gateway enhanced slave station is to take advantage of
using multiple TDM channels and/or TDMA channels concurrently, and
some of those are on multiple different satellites, then it may
have either an antenna that supports transmission and reception
concurrently to those multiple satellites, or multiple antennas
(and necessary associated RF electronics with each antenna, e.g.,
Low Noise Amplifier, and Block Up-converter), each pointed to the
appropriate satellite, if such antenna direction diversity is
required for the satellites used.
Transmitting via TDMA to Secondary Gateways
Using any of the above mentioned methods, the multi-gateway
enhanced slave station (VSAT) may now engage with and receive
digital content or user traffic from multiple other TDM channels
and multiple other gateways. To send TDMA burst transmission to
these other gateways, however, the slave station must be sure, or
else the overall implementation of the multi-gateway networking
system of which the slave station is a part must be able to assure
without any extra effort by the multi-gateway enhanced slave
station, that the slaves TDMA burst transmitting schedule is
appropriate for reception of the slaves transmission bursts by any
other applicable gateway's TDMA burst receivers. Given that each
other gateway is at a separate location and may have only its own
local timing reference, which is not necessarily perfectly adjusted
to be aligned with the master station's overall network clock
reference, this is not a trivial problem. Therefore, for the
multi-gateway enhanced slave station to engage with other gateways
for the purposes of exchanging interactive traffic, or any traffic
that requires transmission to other gateways, additional
considerations are required. However, there are multiple options
for accomplishing this objective. These optional methods include:
a) Having each slave station maintain separate timing advance
values to use with its burst transmitters, for each different
gateway to which it wishes to communicate. They will be applied
relative to slave station's local construction of the master
station's network clock reference, where each slave station obtains
the necessary information to determine these different timing
advance values by using a combination of highly accurate transit
time information for the TDMA carrier signals. These transit times
for determining these timing advances may include: (1) that between
itself and the satellite for those TDMA channels; (2) that between
the same satellite and the master station; and (3) that between the
same satellite and each secondary station to which it wishes to
transmit. All of this is very dynamic information, given the
continual relative motion of the satellite, thus it places a fairly
large burden on the slave station. This option is also limited
because it means that a slave station cannot transmit using a
single burst transmitter, the same message or data packet to
multiple gateways at the same small instant of time. b) Having the
master station determine and inform each slave station of the
various timing advance values it must use to transmit to the other
gateways in the network, which the master station may be able to
accomplish through knowing these same three transit times as
described in a), either by measuring them directly or by
calculating them from accurate position information about each
slave station, gateway station, the applicable satellites and
itself The burden this places on the master station may be eased by
having each secondary gateway station take responsibility for
measuring its own transit delay to the satellite, and independently
providing that information to the multi-gateway capable slave
stations. However, this approach is also subject to the same
functional limitations as above regarding concurrent transmission
in a small instant of time, if the enhanced slave station has only
one burst transmitter. c) Having each secondary gateway station
determine for itself and maintain a single timing advance (or
retard) which will assure that if the slave stations are suitably
aligned in their timing advances for burst transmissions to the
master station, that those same TMDA transmissions can be received
properly at each secondary gateway. This is a preferred option
since it puts only a small burden on the secondary gateways to
maintain this single timing advance (or retard) for itself. It also
allows each gateway, including the master station, to receive the
same TDMA burst transmissions from any given burst transmitter in
any slave station (enhanced or not) as long as each such gateway is
equipped with sufficient numbers of frequency agile TDMA burst
receivers, or simpler types of burst receivers, for that number of
TDMA channels used by those slave stations and where the secondary
gateway is in the footprint of the satellite used for those TDMA
channels. d) Having the master station perform for the secondary
gateway stations what is described above in "c" and transmit that
information to the secondary stations. This is technically possible
but may not be as reliable and lacks certain benefits related to
allowing for distributed operation and fast turnovers from the
master station to a specially designated secondary gateway station
(or order list of such), should the master station fail. e) Having
each gateway station be a master station for the TDMA channels that
it receives. This effectively makes the multi-gateway network into
multiple separate TDM/TDMA networks. It provides some desired
capabilities of the present invention and therefore may be
considered part of it, to the extent that, as described below, it
extends beyond the trivial option of simply implementing multiple
networks. Thus, within the context of configuring each slave
station to communicate with multiple gateway stations, this method
may be enhanced as follows: i) Each enhanced slave station logs on
to its default gateway station by listening to the default (i.e.,
pre-configured) carrier frequency for that station's TDM control
channel, using the appropriate demodulation and decoding
parameters. ii) The enhanced slave station can receive information
about other TDM channels as discussed above and perhaps receive
user traffic or digital content downloads from many of others TDM
channels (but must discriminate and filter out all Layer 2 and
possibly Layer 3 packets pertaining to TDMA network control and
timing and other such unique control messages that might be in
conflict with those transmitted by its current master station,
while it continues to listen to the other information on those
channels). iii) If this enhanced slave station wishes to transmit
to a different gateway station than its current master/default
station, it can either re-tune the TDM receiver that it is using
for reception of the TDM control channel from its current master
station to a TDM control channel of another gateway acting as a
master, or, it must change how it is filtering network control
information so as to filter-out the information coming from current
master and start listening to and responding to the control
information from another gateway acting as new master. This would
likely require logging off the current master and logging on to the
new master gateway so that orderly tracking of such changes occurs
and do not appear to be failure events at slave stations. Thus the
"enhanced" slave station, in this embodiment, can only transmit to
one master station at a time, and most likely must also log off and
log on each time it wishes to transmit to a different master.
Methods "a" and "b" above can be accomplished (assuming the master
station already has the ability to determine the timing advance
that should be used by slaves for transmissions to itself) by
measuring or calculating the following: M=the transit time between
the master station and the applicable satellite G=the transit time
between secondary gateway of interest and the same satellite
And then, the timing advance required for TDMA transmission to the
secondary gateway of interest is calculated for a slave as an
adjustment to the timing advance used for transmission to the
master as follows:
Timing_Adv_for.sub.--Tx_to.sub.--2.sup.nd_Gateway=Timing_Adv_for.sub.--Tx-
_to_Master+(G-M)
However this approach works only if the burst receivers at the
secondary gateway of interest are perfectly aligned to the same
timing reference as those of the master station, as if the two were
one. Due to the distance between them the gateways will have to
undertake additional measures which a person skilled in the art can
implement, such as using a common external timing reference (e.g.,
a GPS timing reference and/or a Network Time Protocol timing
reference).
Method "c" above can also be accomplished by knowing those same two
transit times above (M & G). The timing advance (or retard)
required to align the secondary gateways burst receivers to the
burst plan relative to the timing applied for the master TDMA burst
receivers, is calculated as: 2*(G-M). This additional advance (or
retard) must then be applied relative to the network clock
reference and any advance (or retard) applied for the master TDMA
burst receivers relative to the network clock reference. The
secondary gateway may simply construct a local version of that
network clock reference based on received network clock reference
messages. The timing advance relative to this locally constructed
network clock reference (which is subject to the transit delay
between the master and the secondary gateway) is calculated as:
G+M+2*(G-M)+Master-Receiver-Advance/Retard.
Method "d" can be accomplished the master station making these
measurements or calculations and supplying them to the secondary
gateways, however the additional delay in distributing that
information to the secondary gateways will make this method less
accurate.
Method "e" is fundamentally different in nature and is fully
described above.
Note that it is common practice for such one-way transit delays
(e.g., the values of M and G above) to be measured by having the
ground station measure the round-trip transit time between itself
and the satellite, and then divide by two. It is common for master
ground stations to have this ability and anyone skilled in the art
can add such capabilities to a ground station.
Concurrency in Transmission and Reception
Note that the term "concurrently" in the world of digital
communications can be inexact and may be applied in the case of
multiplexing and/or very fast switching. Thus, considered above are
various options that will or may yield the desired effect of
concurrent transmission to multiple gateways, depending on the
speed at which such "switching" occurs.
Also note that while it has been assumed that the multi-gateway
enhanced slave stations, devised for use in either star topology
networking or mesh topology networking, need not have multiple and
separate physical TDMA burst transmitters, it may be possible and
desirable to have them in some instances. Nonetheless, the
concurrent transmission to multiple gateways over multiple
different TDMA channels can be achieved even with just one TDMA
burst transmitter, because these devices are very frequency agile,
with sub-millisecond agility in some cases.
Likewise, concurrent reception at a slave station from multiple
gateways could in theory be achieved by "frequency hopping" of the
frequency carrier or band at which a single TDM receiver in the
slave station is tuned. This is not well suited to TDM (continuous
mode) receivers. While it would be a "slow frequency" hopping, and
may not appear to the users as "concurrent" it may be considered
one possible implementation option for concurrent reception from
multiple gateways.
Formation of Secondary Gateway Stations
FIG. 6 shows the detailed implementation of a secondary gateway
station, which is derived from the same basic components as a
master station, but having all its functions related to network
control particularly those pertaining to controlling the timing for
the network and the slave stations' burst transmitters disabled.
Thus it does not provide a network clock reference to the slave
stations. Instead it has only a local clock reference (which may be
based on the same equipment as a network clock reference). In
addition, as noted above, it may be tied to a GPS or NTP client so
that it receives a common external timing reference with the master
station. The secondary gateway does not need to maintain the
databases and/or tables of a master station. But most importantly
is does not transmit information related to overall network
management and control such as network clock references, logon
responses, burst plans and timing corrections, (instead it has only
gateway management and control functions). Of course, for the
implementation of option "e" above it would not be necessary to
disable all these network control and management functions, since
each secondary gateway continues to function as a master station
for changing sub-set of the slave stations.
However, as disclosed in the separate patent filing, there may be
additional functions that must be attached to the secondary gateway
to enable a very reliable and efficient means for implementation of
option "c" above or other options above, which as noted requires
TDM receivers to receive the master station's network clock
reference information. (See "Enhance Secondary Gateway" section
later below)
Traffic Routing Control and Information
All such secondary gateway stations may however engage in the
distribution of Layer 3 routing management or policy information
and supplementary network management information or information
requests (e.g., via SNMP) also via Layer 3 (e.g., IP packets),
which do not affect network TDMA timing or the calculation or
determination of timing advances used by the slave stations. Such
routing management information will inform the multi-gateway
capable slave stations where to send which classes of traffic.
Alternatively, the master station may be the only station allowed
to send such information, if centralized routing control is
desired. Additionally it is possible that each slave station (but
multi-gateway capable and not) transmit all TDMA communications to
all of the gateway stations, primary and secondary, that can
receive them, and then each gateway station determines, under a
coordinated routing policy plan among all such gateway stations,
which traffic to forward to terrestrial network connections or on
to other gateway stations, or on to other slave stations.
Enhanced Secondary Gateway Apparatus for Communication Among
Gateways
An enhanced secondary gateway apparatus and its implementation are
disclosed below, which allow for gateway-to-gateway
communication.
FIG. 14 shows an embodiment of the secondary gateway station
enhanced with multiple TDM receivers as are necessary to receive
the TDM channel transmissions from other gateways or the master
station including network timing and other network control
information from the master station. This embodiment may also be
used by a master station in an implementation of a multi-gateway
networking system so that the master station may also receive the
TDM channel transmissions from the secondary gateways in a
multi-gateway network.
Enhanced Master Station Apparatus
An enhanced master station apparatus and its implementation are
disclosed below, that allow the master station in a multi-gateway
network to also function as a secondary gateway. In addition the
enhanced master station has the ability to recognize the slave
stations that are enhanced for multi-gateway operation and
therefore provide the additional information they may required from
the master station to discover and the supplementary TDM channels
and use the other gateways in the network (as discussed
earlier).
This apparatus is similar to that shown in FIG. 5 (a typical master
station or primary gateway) but is further enhanced to support
multiple TDM receivers for user traffic or control traffic among
gateways at each such station, similar to that shown in FIG. 14 for
secondary gateways, but now applied to a station that is capable of
being a master. However this enhanced master station has the
additional ability to disable (via software control) all its
network control processes related to establishing the network clock
reference for the network, distributing burst plan information,
handling capacity requests, responding to logons, distributing
timing corrections (large and small) and other network timing
related information, and do so completely and quickly. In addition
is it able to re-enable (via software control) all the same
processes completely and quickly and return to its role as a master
station on a preconfigured (or dynamically determined) TDM control
channel that may also carry user traffic. Thus, effectively, it can
switch itself between being either a master station or secondary
gateway, under a higher-level control process--not currently in
existence in TDM/TDMA networks--in which it engages with other
secondary gateways that are also master-capable in the same
sense.
This higher level control process relies on communication between
all the gateways (including the current master) via one or more
satellites. This communication may be establish by relying upon
pre-assigned (pre-configured) TDM channel carrier frequencies and
frequency bands (unique to each master-capable station) with
associated modulation parameters and FEC encoding parameters, where
those stations communicate with other such master-capable stations
and by having each such station knowing all such assigned
frequencies and transmissions parameters, as well as their own.
Other mechanisms may also be used whereby this information is
distributed initially (and/or edited and updated occasionally) by
distribution from one preconfigured or designated "primary master
station," which all the others will acknowledge as having that
authority with suitable secure authentication applied to such
update messages.
Over these TDM channels the various master-capable secondary
gateways exchange messages to determine which station will be the
current master station for the network. These same TDM channels may
also be used concurrently or at appropriate times for transmissions
to slave stations (both enhanced and ordinary) and either with or
without network control information being distributed to those
slave stations over the TDM channel, depending on whether "master
station status" has been assigned to that master-capable gateway
station.
CONCLUSION
Those skilled in the relevant art will appreciate that the
invention can be practiced with various telecommunications or
computer system configurations, including Internet appliances,
hand-held devices, wearable computers, palm-top computers, cellular
or mobile phones, multi-processor systems, microprocessor-based or
programmable consumer electronics, set-top boxes, network PCs,
mini-computers, mainframe computers and the like.
Unless the context clearly requires otherwise, throughout the
description and the claims, the words "comprise," "comprising," and
the like are to be construed in an inclusive sense as opposed to an
exclusive or exhaustive sense; that is to say, in a sense of
"including, but not limited to." Words using the singular or plural
number also include the plural or singular number respectively.
Additionally, the words "herein," "above, "below," and words of
similar import, when used in this application, shall refer to this
application as a whole and not to any particular portions of this
application.
The above detailed descriptions of embodiments of the invention are
not intended to be exhaustive or to limit the invention to the
precise form disclosed above. While specific embodiments of, and
examples for, the invention are described above for illustrative
purposes, various equivalent modifications are possible within the
scope of the invention, as those skilled in the relevant art will
recognize. For example, while steps are presented in a given order,
alternative embodiments may perform routines having steps in a
different order. The teachings of the invention provided herein can
be applied to other systems, not necessarily the system described
herein.
While specific circuitry may be employed to implement the above
embodiments, aspects of the invention can be implemented in a
suitable computing environment. Although not required, aspects of
the invention may be implemented as computer-executable
instructions, such as routines executed by a general-purpose
computer, e.g., a server computer, wireless device or personal
computer. Those skilled in the relevant art will appreciate that
aspects of the invention can be practiced with other
communications, data processing, or computer system configurations,
including: Internet appliances, hand-held devices (including
personal digital assistants (PDAs)), wearable computers, all manner
of cellular or mobile phones, multi-processor systems,
microprocessor-based or programmable consumer electronics, set-top
boxes, network PCs, mini-computers, mainframe computers, and the
like. Indeed, the terms "computer," "host," and "host computer" are
generally used interchangeably herein, and refer to any of the
above devices and systems, as well as any data processor.
Aspects of the invention can be embodied in a special purpose
computer or data processor that is specifically programmed,
configured, or constructed to perform one or more of the processes
explained in detail herein. Aspects of the invention can also be
practiced in distributed computing environments where tasks or
modules are performed by remote processing devices, which are
linked through a communications network, such as a Local Area
Network (LAN), Wide Area Network (WAN), or the Internet. In a
distributed computing environment, program modules may be located
in both local and remote memory storage devices.
Aspects of the invention may be stored or distributed on
computer-readable media, including magnetically or optically
readable computer discs, hard-wired or preprogrammed chips (e.g.,
EEPROM semiconductor chips), nanotechnology memory, biological
memory, or other data storage media. Indeed, computer implemented
instructions, data structures, screen displays, and other data
under aspects of the invention may be distributed over the Internet
or over other networks (including wireless networks), on a
propagated signal on a propagation medium (e.g., an electromagnetic
wave(s), a sound wave, etc.) over a period of time, or they may be
provided on any analog or digital network (packet switched, circuit
switched, or other scheme). Those skilled in the relevant art will
recognize that portions of the invention reside on a server
computer, while corresponding portions reside on a client computer
such as a mobile or portable device, and thus, while certain
hardware platforms are described herein, aspects of the invention
are equally applicable to nodes on a network.
Also, while processes or blocks are at times shown as being
performed in series, these processes or blocks may instead be
performed in parallel, or may be performed at different times.
Where the context permits, words in the above Detailed Description
using the singular or plural number may also include the plural or
singular number respectively.
The teachings provided herein can be applied to other systems, not
necessarily the system described herein. The elements and acts of
the various embodiments described above can be combined to provide
further embodiments. All of the above patents and applications and
other references, including any that may be listed in accompanying
filing papers, are incorporated herein by reference. Aspects of the
invention can be modified, if necessary, to employ the systems,
functions, and concepts of the various references described above
to provide yet further embodiments of the invention.
Particular terminology used when describing certain features or
aspects of the invention should not be taken to imply that the
terminology is being redefined herein to be restricted to any
specific characteristics, features, or aspects of the invention
with which that terminology is associated. In general, the terms
used in the following claims should not be construed to limit the
invention to the specific embodiments disclosed in the
specification, unless the above Detailed Description section
explicitly defines such terms. Accordingly, the actual scope of the
invention encompasses not only the disclosed embodiments, but also
all equivalent ways of practicing or implementing the
invention.
Changes can be made to the invention in light of the above
"Detailed Description." While the above description details certain
embodiments of the invention and describes the best mode
contemplated, no matter how detailed the above appears in text, the
invention can be practiced in many ways. Therefore, implementation
details may vary considerably while still being encompassed by the
invention disclosed herein. As noted above, particular terminology
used when describing certain features or aspects of the invention
should not be taken to imply that the terminology is being
redefined herein to be restricted to any specific characteristics,
features, or aspects of the invention with which that terminology
is associated.
In general, the terms used in the following claims should not be
construed to limit the invention to the specific embodiments
disclosed in the specification, unless the above Detailed
Description section explicitly defines such terms. Accordingly, the
actual scope of the invention encompasses not only the disclosed
embodiments, but also all equivalent ways of practicing or
implementing the invention under the claims.
While certain aspects of the invention are presented below in
certain claim forms, the inventors contemplate the various aspects
of the invention in any number of claim forms. Accordingly, the
inventors reserve the right to add additional claims after filing
the application to pursue such additional claim forms for other
aspects of the invention.
* * * * *
References